The invention relates to a device and a method for dispersing gas in a liquid.
The dispersion of gases in liquid media is used widely in the chemical industry, for example in hydrogenations, chlorinations or oxidations. Oxygen input is of considerable importance in fermentation processes and aerobic wastewater treatment. Gas is also dispersed in a liquid medium in foam production. In food technology gases are dispersed in high-viscosity liquids, in order for example to produce creams, foam gums or chocolate with an air-filled porous structure (described for example in WO02/13618A2).
The objective of gas dispersion is to input gas into a fluid, preferably in the form of bubbles that are as small as possible, in order to produce a maximally large interface between the gaseous and liquid phases. The larger the phase interface, the greater the mass transfer between gas and liquid, in accordance with Fick's first law.
Gas dispersion here often proceeds in two steps:
1. introduction of the gas into the liquid in the form of bubbles
2. dispersal of the bubbles
The method of introduction, in general by way of nozzles, frits or perforated plates, determines the size distribution of the primary bubbles. The article “Gasdispergierung in Flüssigkeiten durch Düsen bei hohen Durchsätzen” (gas dispersion in liquids using nozzles at elevated throughputs) from Chemie-Ingenieur-Technik, Volume 28, 1956, No. 6, pages 389-395 for example describes what effect parameters such as nozzle width, gas throughput, viscosity and interfacial tension have on the size distribution of gas bubbles, which arise on injection of a gas jet into a liquid from a nozzle.
Dispersal of the bubbles may proceed for example by means of a dynamic or static mixer. While in dynamic mixers homogenization of a mixture is achieved by moving members such as for example stirrers, in static mixers the flow energy of the fluid is exploited: a delivery unit (for example a pump) forces the liquid through a pipe provided with static internal mixer inserts, wherein the liquid following the main axis of flow is subdivided into partial streams, which are stretched, sheared, swirled together and mixed depending on the nature of the inserts. The advantage of using static mixers resides, inter alia, in the fact that no moving parts are present.
An overview of various types of static mixer is provided for example by the article “Statische Mischer and ihre Anwendungen” (static mixers and their applications), M. H. Pähl and E. Muschelknautz, Chem.-Ing.-Techn. 52 (1980) No. 4, pp. 285-291. Examples of static mixers which may be mentioned are SMX mixers (cf. U.S. Pat. No. 4,062,524) or SMXL mixers (cf. for example U.S. Pat. No. 5,520,460). They consist of two or more mutually perpendicular lattices of parallel sheet metal strips, which are joined together at their points of intersection and are placed at an angle relative to the main direction of flow of the material to be mixed, in order to divide the liquid into sub-streams and mix it. A single mixing element is unsuitable as a mixer, since thorough mixing only proceeds along a preferential direction across the main direction of flow. It is therefore conventional to arrange a plurality of mixing elements in succession, each rotated by 90° relative to one another.
The use of static mixers to disperse gas in a liquid is known. WO02005/103115A1 for example describes the use of a static mixer in a method for producing polycarbonate using the transesterification method. To remove monomers and other volatile constituents from the polycarbonate, a blowing agent is added to the polymer melt. When the pressure is subsequently lowered, the blowing agent escapes, foaming the melt. The foam brings about a major increase in surface area, which is advantageous for degassing, i.e. the removal of volatile constituents. An inert gas, such as nitrogen for example, is preferably used as the blowing agent, which inert gas is introduced into and dispersed in the melt by means of a static mixer, for example an SMX mixer.
US2005/0094482A1 and U.S. Pat. No. 5,480,589 describe static mixers for dispersing gases to produce closed-cell foams. A stepped structure for increasing the effectiveness of gas dispersion is not described.
Dispersion of gas in a liquid generally requires greater mixer lengths than the dispersion of liquids.
On the basis of the prior art, the object arises of providing a device and a method for dispersing gas in a liquid, in order to enable more effective gas dispersion than has been described in the prior art. Compared with the prior art, it is intended to achieve a smaller average bubble size at the mixer outlet while maintaining the same mixer length. Alternatively, a smaller average bubble size is to be achieved at the mixer outlet with an identical pressure drop over the entire mixer.
It has surprisingly been found that a static mixer, in which the specific energy input increases in the direction of flow, has a particularly effective dispersing action. Using such a mixer it is possible, with a comparable overall pressure drop, to produce smaller gas bubbles than with a static mixer, in which the energy input is constant over the length of the mixer. Using such a mixer it is likewise possible, with the same overall mixer length, to produce smaller gas bubbles than with a static mixer, in which the energy input is constant over the length of the mixer.